There has long been an appreciation of the importance of osteoporosis in women, and decades of research have yielded a strong foundation of mechanistic and practical knowledge that underlies clinical decision- making. On the other hand, very little information is available to direct the management of osteoporosis in men. The primary goal of this study (MR.OS) is to determine the extent to which fracture risk in order men is related to bone mass, bone geometry, lifestyle factors, biochemical measures, fall propensity, and other variables. This information is essential for understanding the genesis of fractures, and for the formulation of clinical algorithms for detection and treatment of osteoporosis in men. A second goal is to determine the relationships between osteoporotic fracture and two common chronic conditions in older men prostate cancer and osteoarthritis (OA). This will be a multicenter, prospective study of risk factors for vertebral and all non- vertebral fractures in older men (> 65 years). A total of 5,700 men will be recruited in six diverse geographical areas (Portland, OR; Pittsburgh; Minneapolis; San Diego; Palo Alto; and Birmingham), and will be followed for an average of three years. Baseline assessments will include BMD and bone geometry, vertebral morphometry, OA, neuromuscular and visual function, anthropometrics, nutrition, medical history, medication use, serum/urine/DNA collections, and functional status. Repeat measures of BMD and health/functional status will be obtained at two follow-up visits. The final exam will include repeat radiographs of the spine, hips and knees to determine incident vertebral fracture rate, and incidence and progression of OA. Participants will be followed every 4 months by postcard to determine the rates of non-vertebral fracture, falls, and prostate cancer. Data analysis will utilize proportional hazards and logistic regression models, depending on the type of outcome being analyzed. The study will have 90% power to detect RR of 1.35 for vertebral fractures and OR of 1.22 for non-spine fractures. MR.OS will provide unique information concerning osteoporosis and other common disorders, and will provide a basis for the construction of crucial preventative and treatment strategies.